Pin GitHub Actions workflows to a specific version (#494)

Author: ErikSchierboom

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
         with:
           submodules: true